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Charles University in Pilsen - Biophysics dep - Surface Tension This unequal attraction causes the water at the air-water interface to act as a cohesive surface. A water molecule deep within the liquid is pulled equally from all sides net force = zero A water molecule at the surface lacks neighboring water molecules (toward the interface) net force is not zero! and the liquid to shrink to the smallest possible surface area.

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Charles University in Pilsen - Biophysics dep - Surface Tension Solid material - more dense than water Surface Behaviour water’s surface what happens? hydrophilic it has a surface to which water is attracted hydrophobic it has a surface to which water is “afraid” material’s nature

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Charles University in Pilsen - Biophysics dep - Surface Tension hydrophilic The adhesion of water to the surface of this material coats the surface of the object with water. reduces the surface tensioncauses the object to sink hydrophobic The unfavorable interactions between the water surface and the object make it difficult to wet the surface.

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Charles University in Pilsen - Biophysics dep - Surface Tension Two forces now come into play - the energy it would take to overcome this repulsion and the force of gravity. ! If the gravitational force is less than the surface tension then the object will float on the surface of the water. If the force of gravity is strong enough, it will prevail and the object will sink g σ g σ

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Charles University in Pilsen - Biophysics dep - Surface Tension Surfactants Surfactants can interact with water in a variety of ways, each of which disrupts or modifies the hydrogen bonding network of water. organize themselves into 3-dimensional spheres called micelles which have a hydrocarbon core and sulfate groups around the outer surface. "like dissolves like" Class of molecules which contain hydrophobic (non-polar) hydrocarbon "tails" and a hydrophilic (polar) "head" group. Why? Since this reduces the cohesive forces in water, this leads to reduction in the surface tension and our sulfur sinks.

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Charles University in Pilsen - Biophysics dep - Surface Tension Eg: Detergents Without detergent, we can not remove an oily stain from clothing because grease and oil are large, non-polar, hydrophobic molecules. However, the interior core of a micelle is quite greasy just like an oily stain. When we add detergent to our wash water, the oil or grease on our clothes can dissolve in the interior of the micelles and thereby go into solution. The human body does not start to produce lung surfactants until late in fetal development. Therefore, premature babies are often unable to breathe properly, a condition called Respiratory Distress Syndrome. -administration of artificial surfactants virtually eliminates this health problem Human body - lungs The body uses surfactants to reduce surface tension in the lungs.

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Charles University in Pilsen - Biophysics dep - Surface Tension Du Noüy Ring method A ring of thin wire is submersed but slightly under the surface plan The ring is then gently pulled out of the water and the equilibrium state between surface tension and gravity is measured  = F / 2l If F is the force applied at the dynamometer and l the lengt of the wire curled, surface tension is calculated using the equation

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Charles University in Pilsen - Biophysics dep - Surface Tension Dropping method Measurement of the surface tension of a fluid which is posteriorly related with an already known surface tension of a standard fluid The less surface tension, the less mass of liquid will be droped  2 /  1 = m1 / m2 1- known liquid 2- unknown liquid

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Charles University in Pilsen - Biophysics dep - Surface Tension defined as Surface tension - Scientificaly measured in Newtons per meter (N·m -1 ) represented by the symbol σ or γ or T the energy required to increase the surface area of a liquid by a unit area. the force along a line of unit length perpendicular to the surface.